I. Technical Field
The present invention relates to an optical information recording medium that makes it possible to record and reproduce an information signal by irradiating a thin film formed on a substrate with a high-energy light beam, such as a laser beam or the like, and a method for manufacturing the same.
II. Description of the Related Art
Phase-change type recording media are known as media that makes it possible to record and reproduce a large amount of information with high speed. In phase-change type recording media, recording is performed by utilizing a phenomenon that a recording material thereof is changed to an optically distinguishable state due to heat generated by locally irradiating the recording material with laser light. Phase-change type recording media enable random access as required and have excellent portability, and therefore, are becoming more and more important in recent years. For example, there is an increasing demand in various fields, such as recording or conservation of private data, video information, or the like via computers, the medical field, the academic field, replacement of home video tape recorders, and the like.
At present, larger capacity, higher density, and higher speed are required for phase-change type recording medium as the performance of applications and image information are improved.
Conventionally proposed medium types include rewritable media in which information can be written and erased many times, and write-once media that can be written once. In general, most write-once media includes a smaller number of layers than those of rewritable media, and therefore, are easy to manufacture, i.e., can be manufactured at a lower cost. Write-once media are also convenient when the user wants to write data that is not to be damaged, since they are not rewritable. Therefore, there is a large demand for write-once media, which provide long-life conservation and high reliability, in archival applications.
As a write-once recording material, several oxide materials have been proposed. For example, it has been disclosed that recording materials obtained by diffusing Te particles into oxide base materials, such as GeO2, TeO2, SiO2, Sb2O3, SnO2, and the like, provide high sensitivity and large signal amplitudes (see Japanese Unexamined Patent Publication No. S58-54338). For example, it is known that recording materials containing Te—O—Pd as a major component exhibit large signal amplitudes and considerably high reliability (see International Publication WO98/09823). These Te—O—Pd-based recording materials have a recording mechanism that is considered as follows. The Te—O—Pd film after formation is a composite material in which Te—Pd, Te, or Pd is uniformly diffused as fine particles in TeO2. After irradiation with laser light, melted Te, Te—Pd, or Pd is precipitated as large crystal particles, leading to a change in optical state. Such a change can be detected as a signal.
On the other hand, in recent years, as the amount of information increases, there is a demand for a further increase in recording density of optical information recording media. Therefore, it is necessary to develop a recording medium that can enable high-density recording using a shorter wavelength/higher NA optical system (particularly, blue-violet laser light).
Therefore, it has been proposed that a dielectric layer is additionally provided to a recording layer to increase a reflectance difference so as to use the above-described Te—O—Pd recording material in a blue-violet wavelength region (see Japanese Unexamined Patent Publication No. 2002-133712). Also, in general, a dielectric material is provided on both sides of the recording layer for the purpose of protecting the recording layer and increasing the reflectance difference, and a reflection layer is provided between a substrate and a dielectric layer that is located on a side opposite to the laser incident side of the recording layer for the purpose of effectively diffusing heat generated in the recording layer in the thickness direction to increase the reflectance.
Also, in order to further increase the recording density, there are multilayer media having a plurality of layers into which information is recorded. In such media, when data is recorded into an information layer that is located farther from the light incident surface, laser light needs to be transmitted through an information layer closer to the light incident surface. Therefore, the information layer closer to the light incident surface needs to secure satisfactory signal quality, while it maintains a high transmittance with respect to laser light. The material containing Te—O—Pd as a major component contains TeO2, which is substantially transparent, as a base material, so that it is easy to increase the transmittance of the film, and therefore, can be appropriately applied to multilayer optical information media in which information can be recorded onto multiple information layers from one side.
An example in which four information layers are fabricated using a Te—O—Pd recording material has been proposed (see International Publication WO2004/027770). In this case, a reflection layer is not used for three recording layers closer to the laser incident side, and each of the recording layers is sandwiched by dielectric layers, where the dielectric layer is made of ZnS—SiO2. In this case, however, the jitter of recorded signals is disadvantageously low. The reason is considered that heat generated in the recording layer is diffused in the in-plane direction because it can not be diffused in the thickness direction, so that the edges of recorded marks are blurred.
As described above, in phase-change type recording media, a large amount of heat is generated in the recording layer during recording. Therefore, thermal diffusion occurs in the in-plane direction of the recording layer, so that the edges of recorded marks become unclear.
A heat radiation layer is typically provided so as to diffuse such heat. The heat radiation layer is made of a metal that has a large level of light absorption with respect to laser light. On the other hand, in the case of media having multiple information layers, the transmittances of layers closer to the light incident surface need to be increased so as to secure appropriate reflectances of all of the layers. Therefore, it is difficult to select a material that is capable of reducing light absorption while securing heat radiation performance.
Also, in order to increase signal quality and reliability, an information layer typically includes multiple layers, i.e., a heat radiation layer, a dielectric layer, a recording layer, and a dielectric layer, so that the material cost and manufacturing cost of an optical information recording medium increase.
Further, typically, the heat radiation layer is made of an Ag alloy, and the dielectric layer is made of ZnS—SiO2. Accordingly, corrosion occurs due to a reaction of Ag and S. As a result, when an Ag alloy is used, there is a problem with reliability.
Therefore, there is a strong desire for a large-capacity optical information recording medium that has a high level of long-term conservation reliability and low manufacturing cost, and simultaneously achieves both high signal quality and a high transmittance.